The present invention relates to fire suppressing foam compositions, and more particularly to a film forming foam for use as a fire extinguisher and in other suitable applications.
A number of fire suppressing compositions are known in the fire fighting industry. Depending on the ingredients of the compositions, they are generally divided into chemical foams and mechanical foams. The purpose of covering the fire surface by foam is to form a blanket which preferably forms a homogeneous mask of minute bubbles which ideally resist separation and rupture caused by winds, flame, etc.; which is capable of resealing itself in the event of disturbance to this established foam blanket; and which are liquid enough to flow around objects, reaching and covering areas which might ignite or which are on fire. The foam blanket ideally prevents oxygen supply to the combustion area or, in the case of a flammable liquid spill, covers the liquid preventing hazardous vapor production, as well as possible ignition of the flammable liquid. The benefits of the foam blanket can be contributed, in part, to the fact that the foam bubble has a high water content and creates a cooling effect on heated surfaces. The amount of moisture contained within the foam is usually measured by the foam drainage time--foams with a high moisture content drain at a faster rate of speed than foams with a low moisture content.
Chemical foams are generally produced by a chemical reaction of alkaline salt solution (usually sodium bicarbonate) with an acid salt solution (generally aluminum sulphate) which solution forms a carbon dioxide gas which is trapped in the bubbles of aluminum hydrate with a foaming agent (generally protein hydrolyzate). Due to the nature of the ingredients used to produce chemical foams, the two solutions, that is aluminum sulphate and sodium bicarbonate, have to be kept in separate containers and are mixed only immediately prior to their use for extinguishing a fire. The drawbacks of chemical foams are associated with toxicity of the foams, their corrosive affect on metals and continuous reaction of the ingredients. Such, the ingredient continues until all solution is consumed as a form, which may sometimes result in over applying and "boiling over" of flammable liquids. Also, if the discharge nozzle of fire extinguisher becomes clogged, the result in pressure of the continuous reaction may cause an explosion in the fire extinguishing equipment.
Mechanical forms are produced by aeration of a foam solution to cause entrapment of air within the foam bubbles.
The mechanical foams used are protein foams, fluoroprotein foams, synthetic detergent foams, and aqueous film forming foams (afff).
Protein foam is usually manufactured by alkaline or acid hydrolyses of either vegetable or animal proteins which form a hydrolyzed protein solution. Iron salts are added to provide heat resistance and mechanical stability to the foam bubbles. The protein type mechanical foams are most effective on flammable liquid fires involving hydrocarbon fuels when the temperature of the liquid exposed to the fire does not exceed 250 degrees fahrenheit or when the liquid is in storage tanks. The draw backs associated with protein mechanical foams comprise, entirely, incompatibility of the foam with dry powder fire extinguishing agent, its limited effectiveness in comparison with other types of foams when applied with foam monitors to flammable liquids with low flash points such as gasoline, polar solvents and alcohol, as well as limited ability to achieve total surface covering in difficult to reach places. Fluoroprotein foams were found to be effective for some surface application to tank fires due to their increased resistance to hydrocarbon fuel saturation, their ability to secure the fuel surface against flash back and their superior resistance to radiant heat and over-head water application. To produce fluoroprotein mechanical foam, regular protein based foam liquids and certain fluorinated surfactants are combined.
Synthetic detergent or waiting agent types of mechanical foams are characterized by their significant expansion rate (approx. 20 to 1 as compared to 8 to 1 or 10 to 1 expansion ratios for protein or fluoroprotein foams). The synthetic detergent foam has good fluidity but low stability and rapid drainage time, as well as little radiant heat resistance and rapid dissipation. Synthetic detergent foam liquids comprise surfactants, foam stabilizers and freezing point depressants. The synthetic detergent foams (high expansion synthetic detergent foams) provide an insulating shield from the heat and allows fire fighters to breath and function in the foam mask, if necessary.
Aqueous film forming foam uses fluorocarbon surfactants and various foam stabilizers. It can be used with fresh water or salt water, resists brake down by dry chemical agents. The aqueous film forming foam has low viscosity and surface tension which allows it to spread over the fuel surface rapidly and extinguish shallow or deep fuel spill fires. The major draw back of this type of foam is that it has a rapid drainage time over which may cause the flammable liquid to be exposed to potential ignition, once the foam has drained away.
The present invention contemplates elimination of the drawbacks associated with prior art by providing a mechanical foam which is effective on hydrocarbon fuels, alcohol and polar solvent and flammable liquids.